Anodic passivation of wetted wall vessels



Nov. 9, 1965 c. E. LOCKE ANODIC PASSIVAIION OF WETTED WALL VESSELS FiledNov. 13, 1962 3 Sheets-Sheet 1 POTE/l/T/AL CUAUTDZLEQ /?0 CUEPENI'INVENTOR. 614m 6. Lac/ 5- 47701VEY United States Patent 3,216,916 ANonrcPASSIVATION on warren WALL vnssnrs Carl E. Locke, Ponca City, Okla,assignor to Continental Oil Company, Ponca City, Okla, a corporation ofDelaware Filed Nov. 13, 1962, Ser. No. 237,255 4 Claims. (Cl. 204196)The present invention relates to anodic passivation of corrosible metalsand more particularly, but not by way of limitation, relates to theanodic passivation of vessel Walls over which a corrosive fluid isflowing.

In industry there are frequently instances in which a corrosive fluidflows over a metallic surface. For example, in one particular commercialapplication phosphor is burned in the center of a relatively large tankto produce desired chemical products. The heat of combustion is severeand the walls of the tank must be protected from the high temperatures.One of the best methods found for cooling the tank involves flowingphosphoric acid downwardly over the interior surface of the tank wallsat a substantially uniform depth of a few inches around the entireperiphery of the tank. Since the phosphoric acid will then be heatedsubstantially above room temperature, corrosion of the walls of the tankwill be greatly accelerated, even if fabricated from stainless steel,and the tank will soon be corroded beyond usefulness. In order to combatthe corrosion, the tank could possibly be coated with a highlyresistance ceramic or the like. However, in commercial applications thetanks may be as large as -25 ft. in diameter and perhaps 35 ft. tall,and special coatings and other types of construction become economicallyunfeasible.

It is known in the art that so-called anodic passivation methods can beused to greatly retard corrosion in cases where a fluid is standing in avessel. When using this method, the metallic surface to be protected ismaintained at a constant potential with respect to a reference half-cellor electrode by passing a current from the metallic surface to a cathodedisposed in the corrosive fluid. In this case, the metallic surfacecorresponds to an anode, from which the system gets its name, and thecorrosive fluid serves as an electrolyte for conducting the currentfromthe anode to the cathode. However, anodic passivation systems haveheretofore been applicable only in cases wherein a vessel is eithercompletely filled, or partially filled to a constant level, with thecorrosive fluid such that the fluid is continually in contact with theentire surface of the vessel which is subjected to corrosion. Thecathode is then merely a single, elongated rod of conductive materialdisposed at some convenient point Within the fluid standing in thevessel.

In accordance with the present invention, a method and apparatus areprovided for passivating a metallic surface against corrosion by a fluidflowing over the surface. In general, the method comprises the steps ofdisposing an elongated cathode generally parallel to and spaced from themetallic surface a distance less than the depth of the fluid flowingover the surface such that the cathode will be disposed Within theflowing fluid, disposing a reference electrode within the fluid and inclose proximity to that portion of the metallic surface which is nearestthe reference electrode, and then maintaining the metallic surface at apotential with respect to the reference electrode that is within thepassive region by passing a controlled, direct current from the metallicsurface to the elongated cathode. The present invention alsocontemplates in combination with the wetted wall vessel, a novel cathodeconstruction comprising, in its more specific form, a horizontallydisposed conductive cathode ring uniformly spaced 32%,916 Patented Nov.9, 1965 ice from and extending around the walls of the vessel. Thecathode ring is electrically insulated from the walls of the vessel bynovel connection means and is connected to an electrical source by anovel connector which passes through the wall of the vessel. A referenceelectrode is then located in such a manner as to monitor the maximumpotential of the vessel wall and is passed through the vessel wall andinsulated therefrom by a novel coupling. Therefore, it is an importantobject of the present invention to provide a method for passivating ametallic surface over which a corrosible fluid is flowing.

It is also an important object of the present invention to provide awetted wall tank which is protected from corrosive fluid flowing downthe walls of the tank.

Another object of the present invention is to provide a novel cathodeconstruction for providing anodic protection to a vessel havingvertically disposed walls and a corrosive fluid flowing down the surfaceof the Walls.

Another object of the present invention is to provide a novel cathodefor use in anodic passivation systems for protecting the walls of a tankagainst corrosion.

Yet another object of the present invention is to provide an improvedand economical device for passing a reference electrode through the wallof the vessel.

Another object of the present invention is to provide a device forpassing an electrical lead through the wall of a tank and connecting thelead to a cathode disposed within the vessel. 7

Another object of the present invention is to provide a device forcoupling a ring cathode within a vessel and maintaining the ring cathodeelectrically insulated from the vessel.

Another very important object of the present invention is to provideapparatus of the type described and having the objects and advantages,herein set forth which may be economicallymanufactured and assembled,and which has a long service life.

Many additional objects and advantages of the present invention willbeevident to those skilled in the art from the following detaileddescription and drawings wherein:

FIG. 1 is a plan view of a device constructed in accordance withthepresent invention;

FIG. 2 is a sectional view taken substantially on lines 2-2 of FIG. 1;

FIG. 3 is a sectional view taken substantially on lines 33 of FIG. 1;

FIG. 4 is a sectional view taken substantially on lines 4-4 of FIG. 1;

FIG. 5 is a sectional View taken substantially on lines 5-5 of FIG. 1;and,

FIG. 6 is a plot of the potential of the anode against the currentflowing from the anode tothe cathode of a typical anodic passivationsystem.

Referring now to the drawings, and in particular to FIGS, 1 and 2, atank constructed in accordance with the present invention is indicatedgenerally by the reference numeral 10. The tank 10 has cylindrical,vertically disposed walls 12 and a bottom 14. An annular fluid channel16 or other suitable means is disposed around theupper end of the tankfor distributing a fluid 18 in such a manner that the fluid 18 willspill over the upper edge 20 of the tank and flow downwardly over theinterior surface of the walls 12 at a substantially uniform depth, asillustrated in FIG. 2 at 22. As in the particular commercial applicationheretofore described, the fluid 18 may be phosphoric acid and is used tocool the walls 12 as phosphor is burned generally at bottom center ofthe tank 10 to produce desired products of combustion. The specificmanner in which the fluid 18 is flowed downwardly over the interiorsurface of the tank walls 12 and the manner in which the combustiblematerials are combined and combustion supported, as well as the mannerin which the desired products of combustion are removed, do notconstitute a significant part of the present invention and accordinglyare not illustrated in detail.

A plurality of cathode rings 26 are generally horizontally disposed anduniformly spaced from th walls 12 of the tank around the entirecircumference. The cathode rings may be fabricated from any of themetals found suitable for this purpose by the various workers in theart, such as platinum or stainless steel. In most cases the latter metalwill be preferred because of economic considerations. The cathode rings26 are vertically spaced one from the other in accordance withconsiderations hereafter discussed in greater detail. However, each ofthe cathode rings 26 is of substantially identical construction and eachis supported by a plurality of electrically insulated support mounts,indicated generally by the reference numeral 28.

Referring now to FIG. 3, each of the support mounts 28, which are ofidentical construction, is comprised of a stud 30 which may be threadedinto a plate 32 which in turn is welded to the interior surface of thetank walls 12. A Teflon insulating spacer 34 is disposed around the stud30 and between the plate 32 and the cathode ring 26. The spacer 34 has areduced diameter sleeve portion 36 to form an annular shoulder 37, andthe sleeve portion 36 is inserted in an aperture (not referenced) in thecathode ring 26 until the shoulder 37 abuts the cathode ring. Thus itwill be noted that the spacer 34- both physically supports andelectrically insulates the cathode ring 26 from the stud 30 and the tankwalls 12. A Teflon insulating washer 38 is then disposed around theinnermost end of the stud 30 and a nut 40 is threaded onto the end ofthe stud 30 to securely hold the cathode ring 26 in place. Thus it willbe noted that the cathode ring 26 is completely insulated from theelectrically conductive stud 30 and from the tank walls 12 by the Teflonspacer 34 and the Teflon washer 38.

Each cathode ring 26 is electrically connected to a D.C. electricalpower source by an electrical connection indicated generally by thereference numeral 44. The electrical connection 44 may best be seen inFIG. 4 and is comprised of a tubular sleeve 46 which is threaded into atapped bore 48. The tapped bore 48 extends through the wall 12 of thetank and an electrically conductive bolt 50 having a head 52 is passedthrough an aperture 53 in the cathode ring 26, through an inner Tefloninsulating spacer 54, through the tubular sleeve 46 to a point beyondthe exterior surface of the tank walls 12. The inner Teflon insulatingspacer 54 preferably has a reduced diameter portion 56 which is insertedinto the end of the tubular sleeve 46 to insure that the bolt 50 remainscentered within the tubular sleeve 46. The Teflon insulating spacer 54also provides a means for sealing off the fluid 18 from entering thetubular sleeve 46, although in most cases there will be no fluidpressure, as will hereafter be described in greater detail. A suitablemetallic washer 58 is inserted in the outer end of the tapped bore 48and abuts against the outer end of the tubular sleeve 46. An outerTeflon spacer 60 is disposed around the bolt 50 and has a reduceddiameter sleeve insert portion 62 which is inserted into the metallicwasher 58 substantially as illustrated.

A nut 64- is threaded onto the bolt 50 and when securely tightened, thehead 52 of the bolt 50 will be drawn tightly against the cathode ring 26to insure a good electrical connection. The inner Teflon spacer 54 willbe drawn tightly against the inner end of the tubular sleeve 46 toinsure a fluid tight seal. The threaded connection between the tubularsleeve 46 and the tapped bore 48 provides a fluid tight coupling toinsure that the fluid 18 cannot seep through the wall 12 of the tank 10.At the same time, the conductive bolt 50 is electrically insulated fromthe tank wall 12 by the Teflon inner spacer 54 and the Teflon outerspacer 60.

The outer end of the bolt 50 is preferably provided with a small tappedbore which receives a screw 66. An electrical lead 68 may then besecurely clamped by the screw 66 against the bolt 50 to insure a goodand permanent electrical connection. The electrical lead 68 may thenextend through a suitable conduit 70 to the source of electrical power,hereafter described, and a weathertight hous ing 72 may be providedaround the entire electrical connection and secured to the tank wall 12by bolts 74 in the conventional manner.

A reference electrode is passed through the tank wall 12 adjacent eachcathode ring 26 and is electrically insulated therefrom by a coupling,which is indicated generally by the reference numeral 82 and illustratedin detail in FIG. 5. The coupling 82 comprises a metallic insert 84which is threaded into a tapped bore 86 extending through the tank wall12. A bore 88 extends through the insert 84 and a tapered shoulder 90 isformed Within the bore 88 by a counterbore 92 which is tapped at 94. ATeflon insulating sleeve 96 has a center bore 98 for closely receiving asuitable reference electrode 80. The external diameter of the insulatingsleeve 96 at the inner end 100 is such as to be snugly received withinthe bore 88. The Teflon sleeve 96 also has an enlarged center portion102 which forms an annular shoulder 104 which is tapered to a degreecorresponding to the taper of the shoulder 90 formed by the counterbore)2. The diameter of the enlarged center portion 102 is such as to besnugly received in the diameter of the counterbore 92. The outer endportion 166 of the Teflon sleeve 96 has a diameter correspondingapproximately to the diameter of the inner end portion 100. A second,inner metallic insert 108 is then placed around the outer end portion106 and is threaded into the threads 94 of the tapped counterbore 92.When the second inner insert 108 is tightened, the tapered shoulder 104of the Teflon sleeve 96 will be wedged against the tapered houlder 90 ofthe insert 84- to clamp the relatively resilient Teflon sleeve 96tightly around the reference electrode 80 and thereby provide amechanically secure, fluid tight, electrically insulated coupling.

The outer end of the reference electrode 88 is preferably provided witha small tapped bore for receiving a screw 110 which connects anelectrical lead 112 to the reference electrode 80. The lead 112 may behoused within the conventional electric conduit 114 which is connectedto a suitable electrical housing 116 which is in turn connected to thetank wall 12 by bolts 118.

Referring once again to FIG. 1, a conventional and commerciallyavailable potential controller is represented schematically by thereference numeral 120. The electrical lead 68 from the several cathoderings 26 and the electrical lead 112 from the reference electrode 80 areconnected to the potential controller 120. The tank. Wall 12 is alsoconnected by a suitable electrical lead 122 to the potential controller.The lead 122 may be connected to the wall 12 by any suitable couplingmeans, indicated generally by the reference numeral 124, at almost anypoint, and the coupling means may be protected by a conventional housing126. The potential controller 120, and indeed the broad concept ofanodic passivation, is well known in the art and does not comprise, perse, a part of the present invention. However, it should be noted thatthe reference electrode 80 must be so disposed as to be continuallycovered by the fluid .18, which must. be an electrolyte. The referenceelectrode 80 and fluid 18 then form the conventional half cell whichwill produce a standard potential at reference conditions. The referenceelectrode 88 may be fabricated from any one of several suitablematerials which are known in the art, such as for example, Cr C PtPtO orstainless steel, depending upon which is more economical in view ofinitial cost and service life.

Referring now to FIG. 6, the potential of the anode is plotted againstthe current flowing between the anode and cathode in a typical anodicpassivationsystem. It will be noted that as the potential of the anodebecomes more noble and is changed from a negative potential to apositive potential along the region 130 of the curve, the currentincreases rapidly until the Flade point 132 is reached. The region 139is highly active and the anode will be corroded rapidly. However, as thepotential of the anode is made more noble, i.e., more positive in theconventional current sense, the current drops sharply to a relativelylow value in the passive region 134. If the anode is made still morenoble, so that the passive region 134 is exceeded, the current willincrease sharply once again in the active region 136 and the anode willagain be corroded at an accelerated rate.

In operation, the potential controller 120 compares the potential of thereference electrode 80 with the potential of the tank wall 12 andmaintains the tank wall 12 at a constant potential Within the passiveregion 134 by supplying a direct current, in the conventional sense,through a circuit comprised of the lead 122, the tank wall 12, theelectrolyte fluid 18, the cathode ring 26, the coupling 44 and the lead68. The tank wall 12, which then constitutes an anode, should be quicklylowered to and maintained at the potential within the passive region.Then as the current is passed from the tank wall 12 to the cathode ring26, corrosion will occur at a very rapid rate on the tank wall 12 untila very thin film of protective oxide is formed over the entire surfaceof the tank. During thi phase, a current of considerable magnitude isrequired. However, the protective film formed by the process has arelatively high resistance to electrical currents such that the desiredpotential of the tank Wall 12 can thereafter be maintained by arelatively low current. The high resistivity of the protective filmresults in a high so-called throwing power of the system. In otherwords, each of the cathode rings 26 may be spaced a substantial distanceapart and the surface of the tank walls still maintained at the desiredpotential because the resistance of the protective film is relativelyhigh with respect to the resistance of the fluid electrolyte and thetank wall. Of course the exact vertical spacing of the cathode ringswill vary with each type of installation.

Several important features of the present invention should be noted.First, each of the cathode rings 26 should be spaced from the wall 12around its entire periphery at a distance which is less than the depthof the fluid 18 flowing downwardly over the wall 12 so as to insureconstant and uniform electrical contact between the cathode rings 26 andthe fluid 18. Further, the cathode rings 26 should be uniformly spacedfrom the wall 12 at all points in order to insure that the potential ofany particular spot on the wall 12 will not be less than the minimumpotential at the Flade point 132 nor exceed the potential in the passiveregion 134 and enter the transpassive region 136 which is highly activeand accelerates corrosion. In this respect, it will be noted byreferring to FIG. 5 that the reference electrode 80 is disposed withinthe fluid 18 which flows over the tank wall 12. The reference electrode80 should also be positioned such that it is in close proximity to thatportion of the tank wall 12 which is nearest to a cathode ring 26. Thiswill greatly reduce the possibility of creating so-called hot spotswhere the potential of the tank wall exceeds the passive region andenters the transpassive region so that accelerated corrosion of the hotspot occurs.

It will be appreciated by those skilled in the art that a highly usefultank construction has been disclosed for utilizing anodic passivationmethods to retard corrosion of the walls of the tank. Also, novelcathode and electrically insulated support constructions have beendisclosed which can be economically fabricated and installed. Novelelectrical connections have also been disclosed for passing electricalcurrent through the walls of the tank f3 and through the electrolytefluid to the cathode rings within the tank. Also, a novel insulatingmount has been provided for passing an elongated reference electrodethrough the wall of the tank and positioning the electrode in theelectrolyte fluid.

Having thus described preferred embodiments of the method and apparatusin accordance with the present invention, it is to be understood thatvarious changes, substitutions and alterations can be made thereinwithout departing from the spirit and scope of the invention as definedby the appended claims.

What is claimed is:

1. In combination with a fluid vessel having inclined metallic wallsadapted to have a conductive, corrosive liquid film flow downwardly overthe walls at a depth;

a continuous cathode ring;

nonconductive support means interconnecting the walls and the cathodering for supporting the cathode within the flowing liquid film, saidmeans also supporting the cathode ring in uniformly spaced relation tothe walls around substantially the entire periphery thereof;

a reference electrode disposed in the liquid film and electricallyinsulated from the walls and from the cathode ring except through theliquid film; and,

electrical circuit means connected to the cathode, the

reference electrode and the metallic walls for maintaining the potentialof the walls at a predetermined value with respect to the referenceelectrode by passing a convenional direct current from the walls throughthe liquid film of the cathode ring,

whereby the potential variation of the metallic surface detected by saidreference electrode is used to control by means of the electricalcircuit means, the potential of the conductive surface within thepassive region and corrosion of the surface of the walls by the fluidretarded.

2. In combination with a cylindrical fluid vessel having verticallydisposed metallic walls and adopted to have a conductive corrosiveliquid film flow downwardly over the interior surface of the walls at adepth;

a continuous cathode ring;

a plurality of electrically insulating mounts connected to the walls andto the cathode ring for supporting the cathode ring uniformly spacedfrom the walls and within the liquid film;

a reference electrode extending through the vessel wall and disposed inthe liquid film in close proximity to a point on the interior surface ofthe walls which is nearest the cathode ring;

electrically insulated support means for the reference electrode forsupporting and insulating the reference electrode from the wall exceptthrough the liquid film; and,

electrical circuit means connected to the cathode ring, the referenceelectrode and the walls for maintaining the potential of the walls at apredetermined value with respect to the reference electrode by passing aconventional direct current from the walls through the liquid film tothe cathode ring,

whereby the potential variation of the metallic surface detected by saidreference electrode is used to control by means of the electricalcircuit means, the potential of the conductive surface within thepassive region and corrosion of the surface of the walls by the fluidretarded.

3. The combination as defined in claim 2 wherein each of the pluralityof electrically insulating mounts is comprised of:

a stud member connected to the wall of the vessel and extending radiallyinwardly therefrom, the inner end of the stud member being threaded;

a nonconductive spacer member disposed around the stud member, thespacer member having a portion 7 of reduced diameter adjacent the innerend to form an annular shoulder; an aperture in the cathode ring forreceiving the portion of reduced diameter;

a second insert member disposed around the outer end portion of thesleeve member and threaded into the first insert member, the inner endof the second insert member engaging the outwardly facing shoulder anonconductive washer around the stud member and of the sleeve member,

abutting the inner surface of the cathode ring; and, whereby as thesecond insert member is tightened a nut threaded onto the inner end ofthe stud member against the shoulder, the center portion of the sleevefor clamping the washer against the cathode ring, member will be wedgedagainst the tapered shoulder the cathode ring against the shoulderformed on the in the first insert member and the sleeve member spacermember, and the spacer member against the will be seated against boththe reference electrode wall of the tank, extending therethrough and thefirst insert member whereby the cathode ring will be supported inspaced, disposed therearound.

electrically insulated relationship to the vessel walls. 4. Thecombination as defined in claim 2 wherein the References Cited by theExaminer reference electrode is a cylindrical rod and the electricallyUNITED STATES PATENTS girsigiggted support means for the referenceelectrode com- 393,072 11/88 Marquand 0 e g ggg hhhe ehtehhthe thhehehthe Wehe the 333331152; 32? flitting-3:333:31: 581:3? a first insertmember threaded into the threaded bore, 8/60 Fraser al 2O4 196 the firstinsert member having a bore extendin there- 11/61 Mufhner et 204*147 3049 479 8/62 Preiser et al. 204l47 through, a counterbore to the boreforming an out- Wardly facing tapered shoulder, and internal threadsFOREIGN PATENTS in the counterbore; 204,781 10/23 Great Britain. anonconductive sleeve member disposed in the first 289,586 5/28 GreatBritain insert member and extending therethrough, the 888,767 2/62 GreatBritain sleeve member having a bore extending therethrough for receivingthe reference electrode, an inner end OTHER REFERENCES portion having anexternal diameter to be received Ed l Metallurgia, Septgmber 1954 113-in the bore in the first insert member, a center por- 116,

tion of enlarged diameter to be received in the counterbore in the firstinsert member, and an outer end portion of smaller external diameterthan the external diameter of the center portion to form an outwardlyfacing shoulder on the sleeve member; and,

JOHN H. MACK, Primary Examiner.

MURRAY A. T ILLMAN, WINSTON A. DOUGLAS,

Exammers.

1. A COMBINATION WITH A FLUID VESSEL HAVING INCLINED METALLIC WALLSADAPTED TO HAVE A CONDUCTIVE, CORROSIVE LIQUID FILM FLOW DOWNWARDLY OVERTHE WALLS AT A DEPTH; A CONTINUOUS CATHODE RING; NONCONDUCTIVE SUPPORTMEANS INTERCONNECTING THE WALLS AND THE CATHODE RING FOR SUPPORTING THECATHODE WITHIN THE FLOWING LIQUID FILM, SAID MEANS ALSO SUPPORTING THECATHODE RING IN UNIFORMLY SPACED RELATION TO THE WALLS AROUNDSUBSTANTIALLY THE ENTIRE PERIPHERY THEREOF; A REFERENCE ELECTRODEDISPOSED IN THE LIQUID FILM AND ELECTRICALLY INSULATED FROM THE WALLSAND FROM THE CATHODE RING EXCEPT THROUGH THE LIQUID FILM; AND,ELECTRICAL CIRCUIT MEANS CONNECTED TO THE CATHODE, THE REFERENCEELECTRODE AND THE METALLIC WALLS FOR MAINTAINING THE POTENTIAL OF THEWALLS AT A PREDETERMINED VALUE WITH RESPECT TO THE REFERENCE ELECTRODEBY PASSING A CONVENTIONAL DIRECT CURRENT FROM THE WALLS THROUGH THELIQUID FILM OF THE CATHODE RING, WHEREBY THE POTENTIAL VARIATION OF THEMETALLIC SURFACE DETECTED BY SAID REFERENCE ELECTRODE IS USED TO CONTROLBY MEANS OF THE ELECTRICAL CIRCUIT MEANS, THE POTENTIAL OF THECONDUCTIVE SURFACE WITHIN THE PASSIVE REGION AND CORROSION OF THESURFACE OF THE WALLS BY THE FLUID RETARDED.